This invention relates to a control apparatus applied to a device manufacturing apparatus that prints the pattern image from an original plate to an object to be exposed to light, particularly a scanning-type exposure apparatus ideal for blanket exposure of a large image. The control apparatus controls the drive of two axes, such control requiring a high degree of synchronizing precision and uniformity of speeds.
A control apparatus in which heavy emphasis is placed upon the synchronization of a two-axis drive system has an architecture of the kind shown in FIG. 4, by way of example. A typical example of this control apparatus is described in the specification of Japanese Patent Application Laid-Open (KOKAI) No. 60-169909.
Since the example of FIG. 4 differs from other examples of the prior art in terms expression, the special features of this arrangement will be described. In FIG. 4, G1(s), G2(s) represent closed-loop transfer functions for positional control along the respective axes. If we let P1(s), P2(s) represent the particular characteristics along the respective axes and let C1(s), C2(s) represent the characteristics of compensating devices, then the transfer functions G1(s), G2(s) can be expressed by Equation (1) below: ##EQU1##
Equation (1) makes no mention of loop control along each axis and is an expression from a standpoint focused solely upon the control mechanism for achieving synchronization between the two axes. The expression in two examples of the prior art shown below and the expression of an embodiment of the present invention differ in this respect. For the sake of simplicity, it will be assumed that this specification is directed to a synchronization relationship in which the phase ratio between the two axes is 1:1. In a case where the synchronization control apparatus is applied to synchronized drive systems for the plate stage and substrate stage in a reduced-projection exposure apparatus, it becomes necessary to take the reduction ratio and direction into account. If it is also assumed in this case that a scale conversion is performed between the two axes, a 1:1 synchronization control system can be discussed as the object of interest without sacrificing universality.
The example of FIG. 4 is a so-called master-slave control system. If FIG. 4 is subjected to an equivalent conversion in order to express it in a more easily understandable manner, the result is FIG. 5. Here a first system is driven with a reference value r serving as the input, and a second system is driven by entering a position deviation ex into the second system via a compensating arithmetic unit C3(s), where the deviation ex is between a position output y1 of the first system and a position output y2 of the second system. The position deviation ex is synchronization error and the result ux of the arithmetic control operation performed by the compensating arithmetic unit C3(s) is a synchronization manipulated variable. The position output y1 of the first system G1(s)! is regarded as a command value, and the second system G2 (s)! constructs a feedback loop which follows up the signal y1. This example is one to which a feed-forward path has been added, wherein the slave system is directly influenced by the reference value r. It will be appreciated that this example is intended to improve response at start-up. The feed-forward portion should be considered to be a minor or auxiliary part of the arrangement in terms of architecture of synchronization control, and the basic concept of feedback control resides in the master-slave architecture based upon position control. The example below also is considered in similar fashion with regard to feed-forward.
The example shown in FIG. 6 is described in the specification of Japanese Patent Application Laid-Open (KOKAI) No. 3-252704. Except for the difference in expression indicated in the relations of Equation (1), this arrangement is equivalent to that of FIG. 4 as far as the basic characteristic of synchronization control is concerned. This may be illustrated as follows: Specifically, C3(s) in FIG. 6 is the synchronizing compensating unit, and the output value ux is expressed by Equation (2) below): ##EQU2##
Here the input value to C3(s) is the same as the synchronization error in the example of FIG. 4. Since the control system of FIG. 6 is so arranged that ux is applied to the second system as negative feedback, it will be understood that this arrangement is identical with the master-slave arrangement having the feed-forward path of FIG. 5.
The two examples set forth above differ in terms of the arithmetic architecture but are equivalent in terms of their characteristics. Both are master-slave control systems for two-axis position control. An example different from these is shown in FIG. 7, which illustrates a control system disclosed in the specification of Japanese Patent Application Laid-Open (KOKAI) No. 62-226206. The example of FIG. 7 is fundamentally defective in that the two-axis position outputs y1 and y2 do not follow up the reference input r. However, its basic stance regarding synchronization control will be interpreted and discussed favorably below. That is, the chief feature of this arrangement is that the output signals are joined by being crossed at the output stage of the two-axis drive system. In the master-slave architectures shown in FIGS. 4 and 6, the signal flow is simple, i.e., from the master system to the slave system. In the example of FIG. 7, on the other hand, the basic architecture is such that the two systems are in parallel and interfere with each other on an equal footing. This is referred to as "bilateral" architecture that does away with the master-slave relationship.
Before describing the general problems that arise with the techniques mentioned above, the problems with the bilateral synchronization control configuration will be pointed out. Apart from the example of FIG. 7, which possesses obvious defects, the bilateral configuration is actually preferred in general terms from the standpoint of synchronization control. However, the following two problems frequently arise at the factory where the control system is actually designed:
The first problem is that the overall system is influenced by external disturbance affecting either one of the two systems, and that the resulting oscillation does not readily converge. The second problem is that there are several closed control loops in the two-axis system and it is very difficult to design a compensating device that stabilizes each of the loops simultaneously. The reason why the response to external disturbances is poor is that the dynamics of each system are governed by the slowest oscillation mode, vis-a-vis any disturbance, of the overall system. In the case of the master-slave configuration, the influence of external disturbance can be eliminated owing to the quick-response dynamics solely of the slave system, with regard to synchronization precision. In addition, the master system having the slow-response dynamics is entirely unaffected with regard to disturbances that enter the slave system. These are the advantages of the master-slave system.
The difficulty encountered in designing the compensating devices in the bilateral configuration stems from the fact that it is not possible to simply design each system individually. For instance, assume a case where the compensating device C1(s) is designed based upon the axial characteristic P1(s) in the example of FIG. 7. Even though stabilization of the simple loop containing P1 and C1 may be achieved, no consideration is given to signal components fed back to the simple loop from P1 through C2 and, further, through P2. Such design of characteristics is unsatisfactory. Generally, in order to design the compensating devices of a such a compound system, use of multiple-variable control theory and an associated CAD is desirable. Frequently, however, the result is not only a complicated arrangement for joining signals but also a complicated compensating device, i.e., a higher order arithmetic configuration. Accordingly, this expedient is not easy to realize.
Pointing out the problems of the bilateral arrangement is the same as describing the merits of the master-slave configuration. However, the intention here is not to conclude that the master-slave configuration is almighty. The correct expression would be to say that the bilateral configuration is advantageous in terms of general control theory and the master-slave configuration is advantageous in terms of practical theory.
FIG. 8 illustrates a more universal bilateral control arrangement, though it is not necessarily the most general. Here it is assumed that the servo band of the axial characteristic P2 is sufficiently high in comparison with P1. Further, it is assumed that the four compensating devices C11, C12, C21, C22 calculate the optimum values by employing a CAD to design the appropriate control system. Naturally, the optimization cost function that relates to the synchronization error y1-y2 is essential, and use is made of a weighted quadratic integral, for example, inclusive of velocity deviations along the respective axes. Though illustrating this numerically using examples would be advisable, here we will go no further than to discuss a summary of the results. The compensating device C12(s) for interference from the second system to the first system has a very small gain in comparison with the other compensating devices and the assigned pole zero is sufficiently high, this corresponding to the pole of P2(s). In other words, one arrives at the conclusion that it is possible for the signal path having the compensating device C12 to be eliminated. The reasoning is based on the interpretation that the meaning of artificially combining a fast dynamic characteristic and a slow dynamic characteristic, which are essentially physically separate, is superficial, and that if these characteristics are combined, the effect upon the cost function will be deleterious to the extent that the system of the fast dynamic characteristic is held back by the system of the slow dynamic characteristic. If the path of the compensating device C12 is eliminated in FIG. 8, the signal flow will become unidirectional, i.e., from the first axis to the second axis, and the system obtained will be equivalent to that of the master-slave configuration.
The features of a synchronization control arrangement may be summarized as follows: The universal expression of a two-axis control system in general terms is the bilateral architecture, considered to include the master-slave configuration, which is a special example thereof, as the result of setting the optimum parameters. With the bilateral architecture, however, there are redundant components. This means that a large number of compensating devices must be designed in comparison with the master-slave arrangement. Furthermore, considerable labor and expense is required to stabilize each loop. If there is a clear difference in response between the two axes, the shortest recourse to take in design on the basis of this knowledge would be to construct a control system of the master-slave type, in which the axis having the dominant characteristic is taken as the master system and the axis having the high band characteristic is taken as the slave axis. With a master-slave configuration, designing the compensating devices involves no difficulty whatsoever since the stabilization adjustment is performed only for each axis. Furthermore, if the difference in the response between the two axes is slight, the fact that the controlled system is a two-axis parallel system in which identical operations are capable of being performed means that this difference has little importance in terms of a design problem, the purpose of which is to satisfy the synchronizing specifications.
In the examples mentioned thus far, the common problems exhibit a certain feature. In all three of the examples of FIGS. 4, 6 and 7, the controlled variable is position or a relative position between two axes. In other words, it will be understood that all of the foregoing examples achieve synchronization control in the form of a position control system. Since synchronism is evaluated in terms of relative positions, these examples represent concepts based upon a direct approach. However, in a case where a moving control system of some kind is considered, there are many instances in which controlling velocity is more important and more effective than controlling position. This can readily be understood from the fact that a position signal exhibits a phase lag of 90.degree. as compared to a velocity signal. In regard to synchronization control, the idea of applying velocity-related synchronization in advance has not been clarified heretofore and represents a point overlooked in the prior art.
The prior art mentioned above has a number of problems. Specifically, with the bilateral arrangement that aims at universality, the architecture is complicated and it is difficult to design the stabilizing compensating devices. With the master-slave configuration based upon position control, an operating delay appears in the slave system. Measures for simultaneously realizing synchronization control and velocity control have not been implemented. Attendant problems are higher design cost and more difficult maintenance. Furthermore, in addition to the problems relating to the architecture of synchronization control systems, there are problems relating to suitable ways to synchronize control circuitry and measurement circuitry when implementing the system by hardware.